A conventional servo system employed in an optical disk drive is disclosed in Japanese patent publication No. 49531/1992, and depicted in FIG. 1. With reference to FIG. 1, the conventional servo system includes an optical peak amplifier 1, a gain controlled amplifier 2 having a gain G1, an A/D converter 3, an adder 4 for adding a wobbling signal to the output of the A/D converter 3, a multiplier 5, an equalizer 6, a D/A converter 7, a driving signal amplifier 8, a gain controller 9, and a loop gain detecting circuit 10. In an optical disk drive (not shown) of which the conventional servo system is a part, a laser beam emitted from a laser diode (not shown) is reflected from a reflective surface of an optical disk (not shown) and the reflected laser beam is detected by a photodetector (not shown). An error signal generating circuit (not shown) processes the output of the photodetector to generate an error signal. The gain controlled amplifier 2 amplifies the error signal with the gain G1. The A/D converter 3 quantizes the analog signal output of the gain controlled amplifier 2 into an N-bit digital signal. The adder 4 adds the wobbling signal (which is used for controlling loop gain) to the output of the A/D converter 3. The multiplier 5 multiplies the output of the adder 4 by a gain G2 which is adjusted by the loop gain detecting circuit 10 in such a manner as to cause the loop gain to become "1" at a gain cross point frequency. The equalizer 6 includes filters to suppress noise and minimize steady-state deviation of the error signal. The digital signal output of the equalizer 6 is converted to an analog signal by the D/A converter 7 and then applied to an actuator of the optical peak amplifier 1 via the driving signal amplifier 8, so as to drive an objective lens (not shown) in the proper direction to cause the error signal to become zero, i.e., so as to correct the position of the objective lens and thereby eliminate the error.
In general, the reflectivity of optical disks varies or deviates from a prescribed standard. Thus, the gain of the servo system must be adjusted to compensate for these reflectivity deviations. To this end, the wobbling signal is applied to the loop circuit of the servo system to add a "wobble" component to the output of the optical peak amplifier 1. The loop gain detecting circuit 10 monitors the "wobble" component of the output of the optical peak amplifier 1 in order to detect gain deviations. The gain controller 9, in response to the detected gain deviations, makes the necessary adjustments to the gain G1 of the gain controlled amplifier 2, to thereby remove the deviations in the output of the optical peak amplifier 1 which are due to reflectivity deviations of the optical disk.
FIG. 2 illustrates the change of the error signal upon application of the wobbling signal to the loop circuit. More particularly, it can be seen in FIG. 2 that the error signal varies greatly at the time the wobbling signal is applied to the loop circuit, and thereafter, is gradually damped down. The gain G1 of the gain controlled amplifier 2 is determined during the interval between time t1 and time t2. The "locking state" of the servo system occurs when the error signal is damped down sufficiently for the servo system to be considered stablilized.
With reference now to both FIGS. 1 and 2, it can be seen that the A/D converter 3 of the conventional servo system described above has a fixed (8-bit) resolution. Thus, if the error signal detected during the interval between time t1 and time t2 has an amplitude of 4 volts, the resolution of the A/D converter 3 is 4/(2.sup.8)=4/256 volts.
In operation, after the locking state is detected, the optical disk drive reads data from the optical disk. During normal operation, the error signal (e.g., tracking error signal) has a peak-to-peak amplitude of several hundred millivolts. Therefore, if one resolution step of the A/D converter 3 is set at 4/256 volts as described above, then the digital error signal produced by the A/D converter 3 during normal operation is of relatively low resolution. Obviously, it is desirable to increase the resolution of the A/D converter during normal operation (i.e., after the locking state has been detected) in order to provide a higher resolution digital error signal. However, the cost of improving the resolution of the A/D converter in the conventional manner, i.e., by increasing the number of bits used, is unduly high.
Based on the above, it can be appreciated that there presently exists a need for a digital servo system which eliminates the above-described drawbacks and shortcomings of the presently available digital servo systems. The present invention fulfills this need.